--- a/browser/base/content/browser-tabsintitlebar.js
+++ b/browser/base/content/browser-tabsintitlebar.js
@@ -263,20 +263,16 @@ var TabsInTitlebar = {
       // Reset the margins and padding that might have been modified:
       titlebarContent.style.marginTop = "";
       titlebarContent.style.marginBottom = "";
       titlebar.style.marginBottom = "";
       menubar.style.paddingBottom = "";
     }
 
     ToolbarIconColor.inferFromText("tabsintitlebar", TabsInTitlebar.enabled);
-
-    if (document.documentElement.hasAttribute("customizing")) {
-      gCustomizeMode.updateLWTStyling();
-    }
   },
 
   _sizePlaceholder(type, width) {
     Array.forEach(document.querySelectorAll(".titlebar-placeholder[type='" + type + "']"),
                   function(node) { node.width = width; });
   },
 
   uninit() {

--- a/browser/components/customizableui/CustomizeMode.jsm
+++ b/browser/components/customizableui/CustomizeMode.jsm
@@ -273,18 +273,16 @@ CustomizeMode.prototype = {
               resolve();
             }
           };
 
           Services.obs.addObserver(delayedStartupObserver, "browser-delayed-startup-finished");
         });
       }
 
-      this.updateLWTStyling();
-
       CustomizableUI.dispatchToolboxEvent("beforecustomization", {}, window);
       CustomizableUI.notifyStartCustomizing(this.window);
 
       // Add a keypress listener to the document so that we can quickly exit
       // customization mode when pressing ESC.
       document.addEventListener("keypress", this);
 
       // Same goes for the menu button - if we're customizing, a click on the
@@ -310,18 +308,16 @@ CustomizeMode.prototype = {
       this._wrapToolbarItemSync(CustomizableUI.AREA_TABSTRIP);
 
       let customizableToolbars = document.querySelectorAll("toolbar[customizable=true]:not([autohide=true]):not([collapsed=true])");
       for (let toolbar of customizableToolbars)
         toolbar.setAttribute("customizing", true);
 
       await this._doTransition(true);
 
-      Services.obs.addObserver(this, "lightweight-theme-window-updated");
-
       // Let everybody in this window know that we're about to customize.
       CustomizableUI.dispatchToolboxEvent("customizationstarting", {}, window);
 
       await this._wrapToolbarItems();
       this.populatePalette();
 
       this._setupPaletteDragging();
 
@@ -405,19 +401,16 @@ CustomizeMode.prototype = {
     undoResetButton.hidden = resetButton.disabled = true;
 
     this._transitioning = true;
 
     (async () => {
       await this.depopulatePalette();
 
       await this._doTransition(false);
-      this.updateLWTStyling({});
-
-      Services.obs.removeObserver(this, "lightweight-theme-window-updated");
 
       if (this.browser.selectedTab == gTab) {
         if (gTab.linkedBrowser.currentURI.spec == "about:blank") {
           closeGlobalTab();
         } else {
           unregisterGlobalTab();
         }
       }
@@ -510,36 +503,16 @@ CustomizeMode.prototype = {
       docEl.setAttribute("customize-entered", true);
     } else {
       docEl.removeAttribute("customizing");
       docEl.removeAttribute("customize-entered");
     }
     return Promise.resolve();
   },
 
-  updateLWTStyling(aData) {
-    let docElement = this.document.documentElement;
-    if (!aData) {
-      let lwt = docElement._lightweightTheme;
-      aData = lwt.getData();
-    }
-    let headerURL = aData && aData.headerURL;
-    if (!headerURL) {
-      docElement.removeAttribute("customization-lwtheme");
-      return;
-    }
-    docElement.setAttribute("customization-lwtheme", "true");
-
-    let deck = this.document.getElementById("tab-view-deck");
-    let toolboxRect = this.window.gNavToolbox.getBoundingClientRect();
-    let height = toolboxRect.bottom;
-    deck.style.setProperty("--toolbox-rect-height", `${height}`);
-    deck.style.setProperty("--toolbox-rect-height-with-unit", `${height}px`);
-  },
-
   _getCustomizableChildForNode(aNode) {
     // NB: adjusted from _getCustomizableParent to keep that method fast
     // (it's used during drags), and avoid multiple DOM loops
     let areas = CustomizableUI.areas;
     // Caching this length is important because otherwise we'll also iterate
     // over items we add to the end from within the loop.
     let numberOfAreas = areas.length;
     for (let i = 0; i < numberOfAreas; i++) {
@@ -1168,17 +1141,16 @@ CustomizeMode.prototype = {
   _onToolbarVisibilityChange(aEvent) {
     let toolbar = aEvent.target;
     if (aEvent.detail.visible && toolbar.getAttribute("customizable") == "true") {
       toolbar.setAttribute("customizing", "true");
     } else {
       toolbar.removeAttribute("customizing");
     }
     this._onUIChange();
-    this.updateLWTStyling();
   },
 
   onWidgetMoved(aWidgetId, aArea, aOldPosition, aNewPosition) {
     this._onUIChange();
   },
 
   onWidgetAdded(aWidgetId, aArea, aPosition) {
     this._onUIChange();
@@ -1616,22 +1588,16 @@ CustomizeMode.prototype = {
       case "nsPref:changed":
         this._updateResetButton();
         this._updateUndoResetButton();
         if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
           this._updateTitlebarCheckbox();
           this._updateDragSpaceCheckbox();
         }
         break;
-      case "lightweight-theme-window-updated":
-        if (aSubject == this.window) {
-          aData = JSON.parse(aData);
-          this.updateLWTStyling(aData);
-        }
-        break;
     }
   },
 
   _updateTitlebarCheckbox() {
     if (!AppConstants.CAN_DRAW_IN_TITLEBAR) {
       return;
     }
     let drawInTitlebar = Services.prefs.getBoolPref(kDrawInTitlebarPref, true);